The present invention relates generally to optical inspection systems, and specifically to methods and systems for detecting defects on substrates.
Dark-field systems are well known in the art of optical inspection, particularly for detection of defects on substrates such as semiconductor wafers. Optical signals generated in dark-field inspection are typically characterized by very high dynamic range. The signal depends both on the reflectance of the material in the spot under inspection (as a function of the complex index of refraction) and on spatial variations within the spot. In bright-field systems, the reflectance usually dominates, and the resulting variations in the collected signal are generally no more than about two orders of magnitude. In the case of dark-field detection, however, smooth surfaces lead to almost no collection signal, while surfaces with protruding features may scatter many orders of magnitude more.
Patterned wafers used in producing advanced integrated circuits typically contain regions whose dark-field scattering signals may vary by six orders of magnitude or more. Examples of this phenomenon include scattering variations between the following types of regions:
Scribe lines scatter light differently from pattern regions.
Memory blocks scatter differently from their associated I/O circuitry.
Cache memory of a microprocessor unit scatters differently from the logic area.
A given memory region may have a pitch that generates a strong diffraction lobe toward a detector used to collect the scattered radiation, while the lobes of another region with a different pitch may escape detection.
Bare patches (down to several microns in size) scatter far less than adjoining regions of dense pattern.
The sensitivity of an inspection tool can be optimized for these different regions by controlling both signal acquisition (e.g., laser power and detector gain) and signal processing parameters. It is very difficult, however, to vary the acquisition parameters on the fly, in the process of scanning a single wafer, without reducing the inspection throughput, because the scan speed must generally be reduced in order to avoid artifacts due to rapid changes in the acquisition parameters. Furthermore, defining different regions on the wafer for signal acquisition and processing is a cumbersome task. In the particular case of bare patches (which may be as little as several microns in size, corresponding to a few pixels of the inspection system) between regions of dense pattern, it is impractical to program the inspection system to change its acquisition and processing parameters per region. Although the inspection system may, in principle, be able to adaptively learn the signal processing parameters, it is nearly impossible for the acquisition parameters to adapt within the time span of several pixels. In typical operation of a modern inspection system, this time span is typically no more than tens to hundreds of nanoseconds.
There is therefore a need, particularly in dark-field inspection, for detection systems with a very large dynamic range, in order to collect meaningful signals from both the very dark and the bright areas of the substrate being inspected without on-the-fly adjustment. Achieving a dynamic range greater than 10 or 12 bits (1:1024 or 1:4096) is very difficult, however, with detection systems operating at high data rates (tens to hundreds of mega-samples per second). The dynamic range of the detector itself is limited by the ratio of the saturation power to the minimal detectable signal, typically governed by the noise level of the detector and amplification circuitry. A further limitation is imposed by the restricted bit range of fast analog-to-digital converters.
One possible solution to this problem is to apply non-linear amplification to the output signal from the detector, in order to emphasize the low-amplitude signal range. An approach of this sort is described by Wolf in U.S. Pat. No. 6,002,122, whose disclosure is incorporated herein by reference. The dark-field detector output in this case is processed by a logarithmic amplifier and gain correction mechanism. Although this approach may provide improved visibility of defects in a dark-field image of a substrate, it does nothing to address the fundamental limitation of the dynamic range of the detection system.
Multiple-exposure imaging systems are known in the art. For example, U.S. Pat. No. 4,647,975, to Alston et al., whose disclosure is incorporated herein by reference, describes an electronic imaging camera with expanded dynamic exposure range, based on implementing two succeeding exposure intervals with different exposure parameters. A combined image is then constructed by choosing between the electronic information signals sensed during the two exposure intervals. Typically, the camera is used to combine one exposure of a scene taken with ambient light and another taken under flash illumination. PCT patent publication WO 90/01845, to Ginosar et al., whose disclosure is also incorporated herein by reference, describes image pickup apparatus including multiple image sensors receiving an image at various exposure levels. The sensor outputs are combined to create a single image with widened dynamic range.
It is an object of some aspects of the present invention to provide improved systems and methods for optical inspection, and particularly for detecting defects on a substrate.
It is a further object of some aspects of the present invention to provide dark-field detection systems having enhanced dynamic range.
In preferred embodiments of the present invention, a dark-field optical detection system comprises an illumination source, such as a laser, which scans a substrate under inspection, and two or more detectors for capturing light scattered from the substrate. Preferably, one of the detectors is optimized for high sensitivity, while another is designed to have a high saturation level, typically at the expense of its sensitivity. The light scattered from each point on the substrate is split among the detectors, wherein the respective portions of the radiation directed to the different detectors are not necessarily equal. Preferably, the light is split among the detectors in a consistent manner that is independent of polarization and angle, using a common integrating sphere, for example, to feed all the detectors.
The output of each of the detectors is sampled by its own processing channel. The output of the high-sensitivity channel provides information regarding defects on the substrate in areas of low scattered intensity, while that of the high-saturation channel provides information regarding areas of high scattered intensity. The use of multiple parallel detectors thus allows shot-noise-limited detection in areas with very little available light, without losing the signal due to saturation in bright areas. In this manner, defects on the substrate are detected with far greater dynamic range than can be achieved using single-detector systems, as are known in the art.
In some preferred embodiments of the present invention, spatial filtering is applied to the light scattered from the substrate before the light impinges on the detectors. A key purpose of such filtering is to eliminate constructive interference lobes at certain angles due to repetitive patterns on the substrate, as well as blocking reflections from other bright features on the substrate. Such spatial filtering improves the sensitivity of the detectors to weak scattering signals from local defects. In some of these preferred embodiments, a single spatial filter is used to filter the scattered light that is collected by all the detectors. In other preferred embodiments, at least two of the detectors have their own, separate spatial filters. In this manner, a different filtering characteristic can be applied to the weaker scattered light impinging on the high-sensitivity detector from that applied to the strong scattered light impinging on the high-saturation detector.
Although the preferred embodiments described herein relate to dark-field detection of defects on a substrate, typically a semiconductor wafer, the principles of the present invention are similarly applicable to other sorts of scattering measurements and other detection schemes used in optical inspection of samples of various types.
There is therefore provided, in accordance with a preferred embodiment of the present invention, apparatus for optical inspection, including:
a source of optical radiation, which is adapted to scan a spot of the radiation over a sample, whereby the radiation is scattered from the spot;
a detection system, including at least first and second detectors optically coupled to receive the scattered radiation and to generate respective first and second outputs responsive thereto, the detection system being configured so that the first detector detects variations in the scattered radiation with a greater sensitivity than the second detector, while the second detector saturates at a higher intensity of the scattered radiation than does the first detector; and
a signal processor, coupled to receive the first and second outputs and to determine, responsive to the outputs, locations of defects on the sample.
Preferably, the first and second detectors have respective first and second dynamic ranges, and the signal processor is adapted to process the first and second outputs so as to generate an combined output having a third dynamic range, greater than the first and second dynamic ranges, for use in determining the locations of the defects. In a preferred embodiment, the signal processor is adapted to generate the combined output as a weighted sum of the first and second outputs In another preferred embodiment, the signal processor is adapted to generate the combined output by selecting, at each point as the spot is scanned over the sample, a value of one of the first and second outputs. In still another preferred embodiment, the detection system includes an optical switch, which is adapted to direct the scattered radiation toward either of the first and second detectors in turn, and the signal processor is coupled to drive the switch so as to select, at each point as the spot is scanned over the sample, one of the detectors to which the scattered radiation is to be directed. Typically, the first and second detectors are characterized by respective gains, which are preferably selected so that the sensitivity of the combined output is shot-noise limited.
In yet another preferred embodiment, the signal processor is coupled to process each of the first and second outputs to generate respective first and second defect maps, and to combine the first and second defect maps to determine the locations of the defects on the sample.
Preferably, the first and second detectors are configured to generate the first and second outputs with respective first and second gains, relative to the intensity of the scattered radiation incident thereon, and wherein the first gain is substantially greater than the second gain. In a preferred embodiment, the first detector includes one of a photomultiplier tube and an avalanche photodiode, while the second detector includes a PIN photodiode.
In a preferred embodiment, the detection system includes a beamsplitter, which is positioned to split the scattered radiation between the first and second detectors. Preferably, the beamsplitter is configured to direct a greater portion of the scattered radiation toward the first detector than toward the second detector.
In another preferred embodiment, the detection system includes a diffraction grating, which is positioned to intercept the scattered radiation and to diffract one order of the scattered radiation toward the first detector, and another order of the scattered radiation toward the second detector.
Preferably, the detection system includes an optical element that is adapted to divide the scattered radiation between the first and second detectors in a manner that is substantially independent of a scattering angle and a polarization of the scattered radiation. In a preferred embodiment, the optical element includes an integrating sphere, having an entrance port that is coupled to receive the scattered radiation, and first and second exit ports that are coupled to convey the scattered radiation to the first and second detectors, respectively.
Preferably, the detection system includes at least one spatial filter, which is configured to block a portion of the scattered radiation from reaching the first and second detectors, so as to facilitate detection of the radiation that is scattered from the defects. In a preferred embodiment, the at least one spatial filter includes first and second spatial filters, which are respectively positioned so that the first spatial filter filters the scattered radiation reaching the first detector, while the second spatial filter filters the scattered radiation reaching the second detector. Preferably, the signal processor is coupled to independently control each of the first and second spatial filters, so as to alter the portion of the scattered radiation that is blocked by each of the spatial filters.
Optionally, the detection includes at least one attenuator, which is controllable so as to adjust an intensity of the scattered radiation that reaches at least one of the first and second detectors.
In a preferred embodiment, the apparatus includes a third detector, optically coupled to receive the scattered radiation and to generate a third output responsive thereto, wherein the sensitivity of the third detector is intermediate the sensitivity of the first and second detectors, and wherein the signal processor is coupled to receive the third output and to determine the locations of the defects responsive to the third output, together with the first and second outputs.
Typically, the optical radiation includes coherent radiation, and the detection system is configured so that the detectors receive the scattered radiation in a dark-field mode. In a preferred embodiment, the sample includes a semiconductor wafer on which a pattern is formed, and the signal processor is adapted to map the locations of the defects in the pattern.
There is also provided, in accordance with a preferred embodiment of the present invention, an integrating sphere, including:
an inlet port, adapted to receive radiation;
a spherical body, having an internal surface that is adapted to diffusely reflect the radiation received through the inlet port; and
first and second output ports, adapted to convey the radiation from within the spherical body to first and second detectors, coupled respectively to the ports, the first port having a substantially greater diameter than the second port, whereby a substantially greater portion of the radiation is conveyed to the first detector than to the second detector.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a method for optical inspection, including:
scanning a spot of the radiation over a sample, whereby the radiation is scattered from the spot;
configuring at least first and second detectors so that the first detector detects variations in the scattered radiation with a greater sensitivity than the second detector, while the second detector saturates at a higher intensity of the scattered radiation than does the first detector;
detecting the scattered radiation using at least the first and second detectors so as to generate respective first and second outputs responsive thereto; and
processing at least the first and second outputs to determine locations of defects on the sample.
There is further provided, in accordance with a preferred embodiment of the present invention, a method for processing radiation, including:
collecting the radiation in an integrating sphere; and
coupling first and second detectors to respective first and second output ports of the integrating sphere, the first port having a substantially greater diameter than the second port, whereby a substantially greater portion of the radiation is conveyed to the first detector than to the second detector.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: